Mosquitos and the Diseases they transmit - School Integrated Pest

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Transcript of Mosquitos and the Diseases they transmit - School Integrated Pest

Mosquitos and the Diseases they transmitB-6119 6-02
J. A. Jackman and J. K. Olson*
Mosquitoes are among the most important insect pests affecting the health of people and animals. Biting female mosquitoes not
only irritate people and animals, but they can also transmit many disease-causing organisms.
Annoying populations of mosquitoes can occur any- where in Texas because there are habitats favorable for mosquito species almost everywhere in the state.
To control mosquitoes effectively, it helps to under- stand their life cycle, to be able to identify the various kinds of mosquitoes, and to know what steps work best for the different species and specific locations.
Life histo ry Mosquitoes have four distinct stages during their life
cycle: egg, larva, pupa and adult. The adult stage is free- flying; the other stages are aquatic.
The length of time that a mosquito takes to complete its life cycle varies according to food availability, weath- er conditions and the species of mosquito. Under favor- able conditions, some mosquitoes can complete their entire life cycle in only 8 to 10 days.
Egg One way to identify
the breeding sites of mosquitoes is to find the eggs. Mosquito eggs may be laid in clusters called rafts on the water sur- face. They may also be laid singly on the water surface or in dry areas that are flooded periodi- cally.
When first laid, mosquito eggs are white, but within a few hours they become dark brown to black. The shape and size of mosquito eggs vary, with most being football- shaped or boat-shaped and 0.02 to 0.04 inch long.
In warm water, the eggs may hatch in 2 to 3 days. Some mosquito eggs can remain dormant in dry condi- tions for many months.
Larva Mosquito eggs hatch into long larvae called wigglers,
which are seldom over 1/2 inch long. Wigglers have three body sections: a small head, an enlarged thorax (the middle section) and a long, cylinder-shaped abdomen.
Wigglers live only in water. Most of them feed on microscopic plants, animals and organic debris suspend- ed in the water. They filter the food particles from the water with their brush-like mouth
*Professor and Extension Entomologist, Texas Cooperative Extension, and Professor of Entomology, Texas Agricultural Experiment Station, Department of Entomology; The Texas A&M University System
Life cycle of a mosquito.
Adult mosquito laying eggs.
Mosquito larvae.
parts. The larvae of some mosquito species are predators that feed on other mosquito larvae.
Most mosquito larvae mature in 4 to 10 days, passing through four instars (growth stages) and then transform- ing into the pupal stage. The length of a mosquito’s lar- val development period depends on food, temperature and species.
While feeding or breathing, mosquito larvae assume distinctive positions in the water. For most species, the larva breathes through an air tube located near the end of the abdomen. It projects the air tube through the water surface and hangs head down at an angle to the water surface, with only the tip of the breathing tube coming into contact with the surface of the water.
An exception is the larvae of Anopheles mosquitoes. They lack air tubes and tend to lie flat against the water surface.
One way to control mosquitoes in the larval stage is to apply a surface film of certain petroleum products in standing water. This film disrupts their breathing and kills the larvae.
Pupa The pupal stage is
the transitional stage between the mosquito larvae, which live in the water, and the adults, which live on land.
Mosquito pupae do not eat. They spend most of their time at the water surface and tend to move only when disturbed. Mosquito pupae are sometimes called tum- blers because of their tumbling motion in water when they are disturbed
Mosquito pupae are comma-shaped and, like the lar- vae, breathe through air tubes at the surface of standing water. The front region of the pupa’s body is greatly enlarged, consisting of a fused head and thorax. A pair of respiratory tubes, or trumpets, extends from the back of the thorax and are used at the water surface to breathe air. The pupal abdomen or tail consists of sever- al segments that move freely.
The pupal stage may last from 1 to 10 days, or even more, depending on the temperature and the mosquito species involved. Pupae may also be killed with surface film treatments.
Adult Adult mosquitoes
have wings. Male mos- quitoes feed only on nectar, plant juices and other sources of liquid carbohydrates. Male mosquitoes usually emerge a few hours up
to a few days before the females emerge. The males rest in the vege- tation surrounding the emergence site, waiting for females to emerge.
Female mosquitoes also feed periodically on nectar, plant sap and other sources of plant carbohydrates for energy. However, the females of most species must have a blood meal as a source of protein before they can produce eggs.
Mating usually occurs near the emergence site and coupling occurs quickly in the air. Female mosquitoes can fertilize all of their eggs after a single mating because they can store the sperm internally. Male mos- quitoes usually die shortly after the mating period.
Adult female mosquitoes typically live for about a week to a month, but this can vary depending on several environmental factors. Some species overwinter (spend the winter) as engorged mated females that may live up to 6 months or more.
When female mosquitoes are inactive, they rest in protected areas that are typically dark or shaded, humid, and cool in the summer or warm in the winter.
The mouthparts of female mosquitoes are complex and form a prominent beak or proboscis. When a mos- quito takes a blood meal, it uses its mouthparts to punc- ture the skin of the host and feed directly from the cap- illaries (small blood vessels).
As it feeds, the mosquito injects a small amount of saliva into the wound before drawing blood. The saliva makes penetration easier and prevents the blood from clotting during feeding. In most cases, the itching and swelling caused by the saliva subside within a few hours.
The adult mosquitoes around your home may have come from a breeding site near or far away, depending on the species, wind patterns and the flight habits of the females:
Aedes aegypti breed primarily in and around human habitations and fly short distances, usually only about 200 yards.
Most Anopheles mosquitoes have a flight range of about 1 mile. Psorophora species have flight ranges of at least 5 miles.
Some salt-marsh mosquitoes in the genera Aedes and Ochlerotatus can disperse with the prevailing winds for 20 to 40 miles or more away from the larval development sites.
You need to recognize these flight distances in order to find the source of mosquito problems and choose the appropriate management strategies. If the source of the mosquitoes is not on your property, you may need to
Mosquito pupae.
cooperate with others to be able to control them. In some cases, you may be unable to control them at all.
Id e ntifying mosquitoes All mosquitoes are classified in the order Diptera
(true flies) and in the family Culicidae, which has more than 2,500 recognized species in the world. Adult mos- quitoes are small, long-legged flies that have two wings like most other true flies.
All adult mosquitoes have three characteristics in combination, which separates them from all other flies:
Long, many-segmented antennae A piercing and sucking mouthpart system elongat-
ed into a distinctive beak or proboscis, at least in the females
Scales on the wing veins and margins To control the mosquitoes in your area, first you need
to know what kinds are living there. At least 84 species of mosquitoes are known to occur in Texas. These
species have considerable variation in their larval breed- ing sites, time of day when they bite, and flight dis- tances of the adults. Table 1 provides a summary of this information for some common species in Texas.
Identification of larvae or adult mosquitoes to species is complicated and generally requires considerable expertise and training. If there is a question about the species involved, it is best to send samples to an identifi- cation lab. A mosquito control district, a university, or a pest control operator should be able to help.
G roups of mosquitoes Mosquito species are often divided into groups based
on where the females lay their eggs and where the lar- vae develop. There are different control strategies for each group. The Center for Disease Control and Prevention (CDC) has developed a convenient grouping of four mosquito types according to the habitats in which the larvae generally develop:
Table 1. Biological data on 20 common species of mosquitoes found in Texas
Mosquito Species Larval Habitat(s) Biting Time Flight Range
Aedes aegypti AC C, D under 100 yards
Aedes albopictus AC, TH C, D 100 - 300 yards
Aedes vexans FW, GP, IP C, N 10 - 25+ miles
Anopheles punctipennis WP C, N 0 - 0.25 mile
Anopheles quadrimaculatus FW, GP, LM C, N 0.5 - 1 mile
Culex erraticus WP N 0 - 0.25 mile
Culex nigripalpus GP, FW, DD C 0.5 - 1 mile
Culex quinquefasciatus AC, SCB, GRP C, N 0.25 - 0.5 mile
Culex restuans WP, GRP, DD C, N 1 - 2 miles
Culex salinarius GP, LM, FS, SM C, N 0.25 - 5 miles
Culiseta melanura FS, WP C, N 0.5 - 1 mile
Ochlerotatus (=Aedes) atlanticus WP C, D 0.25 - 0.5 mile
Ochlerotatus (=Aedes) canadensis WP, DD, FS C 0 - 0.25 mile
Ochlerotatus (=Aedes) sollicitans SM C, N, D 5 - 40 miles
Ochlerotatus (=Aedes) taeniorhynchus SM C, N, D 5 - 40 miles
Ochlerotatus (=Aedes) triseriatus TH, AC D 0.5 - 1 mile
Psorophora ciliata IP, RF, GRP C, N 5 - 10 miles
Psorophora columbiae IP, RF, GRP C, N 5 - 10 miles
Psorophora ferox WP C, N 1 - 2 miles
Psorophora howardii WP, Coastal Pools C, N 1 - 2 miles
AC: Artificial containers GP: Grassland pools RE: Rooted emerged vegetation DD: Drainage ditches GRP: Ground pools RF: Rice fields FS: Freshwater swamps IP: Irrigated pastures SCB: Sewage catch basins FW: Flood waters LM: Lake margins SM: Salt marshes WP: Woodland pools TH: Tree holes C: Crepuscular (dusk and dawn) D: Day N: Night
Modified from the original source:
Permanent pool group Transient water group Floodwater group Artificial container and tree-hole group Understanding these groups will help you determine
the source of the mosquitoes and what control measures are likely to help. Table 2 provides a summary of these groups, the more problematic genera and/or species, lar- val breeding habitats and general management approaches.
Managing mosquitoes To control mosquitoes effectively long-term, you need
to use several complementary management techniques, including:
Sanitation — removing food, water, and shelter Habitat disruption — draining the water where
mosquitoes breed Biological control — using mosquito fish, nema-
todes, and Bacillus thuringiensis israeliensis toxin and Bacillus sphaericus
Mechanical control — maintaining window screens and altering building designs
Personal protection — wearing protective, light- colored, loose-fitting clothing; using repellents; and avoiding activities in areas when mosquitoes are active
Chemical suppression — using insecticides against adults and/or larvae
Mosquito control is often complex and expensive, requiring the cooperation of individual homeowners as well as such groups as industry, agriculture, state agen- cies and local governments.
Some communities may have to take an areawide approach to mosquito control and hire permanent con- trol personnel. These areawide programs are sometimes the only solution to a mosquito problem. Areawide mos- quito control programs can provide manpower and expertise that are usually unavailable to homeowners.
In these programs, trained personnel conduct mosqui- to surveys to identify the species, track the population levels and determine appropriate management alterna- tives. Areawide management can provide relief from
Table 2. Mosquito groups, their breeding sites and management suggestions for each group.
Mosquito Genera and/or Breeding Sites General Management Group species Approaches
Permanent Anopheles, some Culex, Standing water that seldom Biological control—using mosquito pool Culiseta, Coquillettidia, dries, edges of ponds, lakes fish, nematodes, and Bacillus
Mansonia and smaller impoundments thuringiensis israeliensis toxin and Bacillus sphaericus
Habitat disruption—draining the water or removing plants
Transient Culiseta, some Culex, Roadside ditches, excavations, Biological control—using nematodes, water occasionally Anopheles, canals, ground pools, catch and Bacillus thuringiensis israeliensis
especially Anopheles basins, storm sewers, clogged toxin and Bacillus sphaericus punctipennis streams, irrigated land
Sanitation—removing food, water and vegetation
Floodwater Aedes, Ochlerotatus, flood plains, salt marshes, Habitat disruption—draining the water Psorophora smaller sites, even animal where mosquitoes breed
footprints Chemical suppression—using insecticides against adults and/or larvae
Artificial Most Aedes—especially Artificial containers, discarded Sanitation—removing food, water, container` Aedes aegypti, Aedes tires, tin cans, flower pots, and shelter and albopictus and Ochlerotatus cemetery vases, roof gutters, tree-hole (=Aedes) triseriatus tree-holes, water caught in Habitat disruption—draining the water
bromeliads and orchids and in small containers other plants
mosquito problems that develop miles away from the affected area.
Laws have been enacted in Texas enabling various groups to form mosquito control districts. The Texas Department of Health in Austin can provide information on how to establish a control district. The Texas Animal Health Commission monitors the mosquito-borne animal diseases in the state.
Homeowners can reduce their local program in many cases. More specific management actions are provided by location in Table 3. Evaluate your area to recognize the likely larval breeding sites for mosquito larvae then take appropriate action.
How mosquitoes affe ct people and animals
Mosquitoes can affect people and animals directly or indirectly.
Direct effects: Mosquito species that feed on blood can annoy people, birds, mammals and other verte- brates. They disrupt outdoor work and recreational
activities. If there are enough mosquitoes in an area, they can cause severe blood loss and slow the growth of livestock.
When they feed, mosquitoes inject salivary fluids into their hosts. These fluids contain chemicals that can pre- vent blood from clotting and break down red blood cells.
The bites often cause mild allergic reactions such as swelling and itching. These reactions may continue to affect the hosts long after the female mosquitoes have taken their blood meals. Although some people may react more strongly to the bites, severe reactions are uncommon.
Indirect effects: Mosquitoes indirectly affect people and animals when they transmit disease organisms to them. Each year worldwide, mosquitoes affect millions of people by transmitting the disease-causing agent (pathogen) of several serious diseases, including encephalitis, dengue, yellow fever, malaria and filariasis.
Although most of these diseases were once common in the United States, none causes serious human prob- lems in the United States today. The most common of these diseases now in Texas is encephalitis, which affects a few people each year.
Table 3. Possible mosquito sources around the home and other property.
Mosquito Sources How to Reduce Mosquitoes
Ponds Stock the pond with fish. Use Bacillus thuringiensis israelensis (such as Mosquito Dunks®). Remove excess vegetation.
Swimming pools Keep water off the cover. Maintain water quality at all times.
Tree holes Fill the holes with sand, or drill a drain hole.
Plastic pools Drain the water when not in use or cover the pool to prevent mosquitoes from laying eggs in the water.
Containers Empty the water. Store the containers in an inverted position. Dispose of the containers. Cover the containers so mosquitoes cannot lay eggs in them.
Bird baths Change the water at least once a week.
Standing water Eliminate it by draining it. Fill in low areas.
Watering troughs Stock the trough with fish. Change the water weekly.
Cooler drains Prevent water from standing in the drain.
Street gutter or catch basins Keep litter and garden debris out of the gutter. Do not overwater the yard.
Cesspool or septic tanks Seal and cover it so mosquitoes cannot lay eggs in it.
Roof gutters Clean them once a year to remove debris.
Irrigated lawns or fields Avoid overirrigation. Drain standing water.
Modified from the original source: UC Pest Management Guidelines.
Another common mosquito-borne disease in Texas is heartworm in dogs, particularly in the humid areas of East Texas and the Coastal Plain.
Mosquitoes rarely cause death directly, but they can do so by a combination of effects, including exhaustion, suffocation, toxemia and blood loss.
Mosquito-borne diseases Mosquito-borne diseases can become a problem when
these four living elements are present: Pathogen — the organism that causes the disease Reservoir — the animals in which the pathogen
lives and which serve as the source of the pathogen for the mosquitoes that transmit it
Susceptible hosts — the people and/or other ani- mals that can be infected by the pathogen
Vectors — the mosquito species that can transmit the pathogen, either mechanically or biologically, from it reservoirs to the susceptible hosts
All four of these living elements must be present for a mosquito-borne disease to continue to occur and cycle in a geographic location.
Encephalitis Encephalitis is an inflammation of the brain caused
by certain viruses transmitted by mosquitoes. Human cases of encephalitis range from unapparent or mild cases to very severe illnesses that can permanently dam- age the central nervous system or, in some instances, cause death. Symptoms include high fever, convulsions, delirium and other central nervous system problems. If these symptoms occur, seek medical assistance quickly.
Birds serve as the primary reservoirs for the viruses that most often cause the disease. Encephalitis occasion- ally becomes a problem when it is transmitted to humans, horses or other equines such as donkeys or mules.
Public health officials often check local and migrating bird populations to see if they have the virus and whether there is a potential for its transmission to humans and animals.
Several types of encephalitis cause concern in Texas: eastern equine encephalitis (EEE), western equine encephalitis (WEE) and St. Louis encephalitis (SLE). The viruses causing EEE, WEE or SLE are normally transmit- ted from bird to mosquito to bird and sometimes from bird to mosquito to human. EEE and WEE can also be transmitted from bird to mosquito to horse. When the incidence of any encephalitis virus increases in bird pop- ulations, it becomes more likely that humans and equines can become involved.
In most cases, the human or equine host is a “dead- end host” for the virus, meaning that the disease proba- bly will not be transmitted from these hosts because they cannot infect mosquitoes.
Similarly, horses may have mild to severe or even fatal infections with EEE or WEE viruses. Horses with the SLE virus show no outward sign of infection.
Birds may die of infection caused by some encephali- tis viruses but not by others. For example, deaths from the EEE virus have been reported in red-winged black- birds, house sparrows, pheasants and emus. SLE virus, however, produces no outward sign of infection in birds.
The various types of viruses are transmitted by differ- ent mosquito species:
EEE virus: Culiseta melanura, a mosquito that breeds in freshwater swamps, is typically involved in the bird to mosquito to bird cycle of the eastern equine encephalitis virus.
Because this mosquito rarely bites humans or horses, other mosquitoes, such as the eastern salt marsh mos- quito, Ochlerotatus (=Aedes) sollicitans, probably transmit EEE virus to humans and horses.
WEE virus: Culex tarsalis and Ochlerotatus (=Aedes) dorsalis are the main mosquito vectors for western equine encephalitis virus, particularly west of the Mississippi River. Other insects, such as the swallow bug (Cimicidae), may also serve as overwintering hosts of the WEE virus.
SLE virus: Members of the Culex pipiens complex, Cx. p. pipiens (the northern house mosquito) and Cx. p. quinquefasciatus (the southern house mosquito), are…